1. Field of the Invention
The present invention relates to radio frequency (RF) power amplifiers and amplification methods. More particularly, the present invention relates to feed forward power amplifiers and methods of using a pilot to align the loops of a feed forward amplifier.
2. Description of the Prior Art and Related Information
A primary goal of RF power amplifier design is linearity over the range of power operation. Linearity is simply the ability to amplify without distortion. This requirement is critical for modern wireless communication systems but it is increasingly difficult to achieve. This is due primarily to the bandwidth requirements of modern wireless communication systems and the use of spectrally efficient modulation formats with high peak power relative to the average power which are placing increasing demands on amplifier linearity.
Feed forward compensation is a well known approach applied to amplifiers to improve linearity by estimating and canceling distortion. In feed forward RF power amplifiers, an error amplifier is employed to amplify only distortion components which are then combined with the main amplifier output to cancel the main amplifier distortion component.
FIG. 1 illustrates a conventional feed forward amplifier design. The system has in RF input and an RF output. The RF signal may be a high bandwidth signal such as a CDMA (Code Division Multiple Access) spread spectrum communication signal or WCDMA (Wide Code Division Multiple Access) signal, or other RF signals.
The system includes a main amplifier 1 and an error amplifier 2. The basic elements also include delays 3, 4 in the main and error paths, respectively, and main to error path couplers 5, 6, 7 and 8. Additional elements not shown are also typically present in a conventional feed forward architecture as is well known to those skilled in the art. The delays, couplers and error amplifier are designed to extract distortion components from the main path and inject out of phase distortion components from the error path into the main amplifier output at coupler 8 to substantially eliminate the distortion component in the main amplifier path.
The performance of a feed forward amplifier may typically be analyzed based on two cancellation loops. The first loop, called the carrier cancellation loop, ideally provides a signal at the output of coupler 7 with the input RF carrier component cancelled and only a distortion component remaining. The second loop is referred to as the error cancellation loop or auxiliary path loop. In the second loop the distortion component provided from coupler 7 is amplified by the error amplifier 2 and injected at coupler 8 to cancel the distortion component in the main path and ideally provide a distortion free signal at the output.
The quality of the distortion estimate (carrier cancellation) is determined by the alignment of the first loop in terms of gain magnitude and phase, if the alignment is controlled by gain and phase adjusters, or the in-phase (I) and quadrature (Q) components of the complex gain, if the alignment is controlled by a vector modulator. The distortion cancellation in turn is determined by the alignment of the second loop in terms of the complex gain. In prior art systems, a pilot 9 is injected into the main amplifier path of the first loop, acting like a known distortion signal. The pilot signal is detected at the feed forward amplifier output by a pilot detector 10 and used to aid the alignment process for the second loop. When the second loop is aligned, the pilot is cancelled. If the second loop is misaligned, residual pilot power will be detected at the output of the feed forward amplifier. The degree of the misalignment is estimated from the measured power of the residual pilot. The alignment of the second loop is adjusted in an iterative manner with the goal of reducing the residual pilot power. Generally, it is desirable to have the feed forward amplifier control system adapt to the optimal settings as fast as possible to minimize the amount of time the amplifier operates at a less than optimal setting.
One difficulty with alignment control algorithms used to adjust the alignment settings (I and Q components of the complex gain) from any initial setting to that which results in the best measured alignment is the difficulty in finding the correct direction of adjustment in the two dimensional (2D) I-Q space. Prior alignment control algorithms typically rely on either the “steepest descent” or the “coordinate descent” algorithms. The steepest descent algorithm adjusts the alignment settings in a direction of the gradient within the 2D I-Q space. Dithering the alignment in orthogonal directions and measuring the changes in measured misalignment provides an estimate of the gradient. The coordinate descent algorithm performs two separate 1D searches along pre-defined orthogonal directions (usually the I and Q axes). The alignments are dithered to determine which direction along the respective coordinates reduces measured misalignment. Both these approaches have disadvantages in practical systems which employ control processors with limited processing power and where fast loop alignment is desired. As a result the desired fast and accurate loop convergence has not been achieved in practical adaptive feed forward systems.
Accordingly, a need presently exists for a system and method for more rapid loop alignment control in a feed forward amplifier system.